Indirectly heated cathode for gas tubes



Nov. 22, 1955 D. v. EDWARDS 2,724,788

INDIRECTLY HEATED CATHODE FOR GAS TUBES Filed Feb. 12, 1952 F O I 1 FIG. 3. *rW A 25 f H a 26 5 27 10 41 2 2 52 IN VEN TOR.

BY DV Edwards.

MM M

H is. ATTORNEY United States Patent INDIRECTLY HEATED CATHODE FOR GAS TUBES Donald V. Edwards, Montclair, N. 5., assignor to Electrons, Incorporated, Newark, N. .l'.

Application February 12, 1952, Serial No. 271,110

4 Claims. (Cl. 313-339) This invention relates to cathode structures for gaseous discharge tubes, and more particularly to a structural organization for indirectly heating heat shielded oxide coated cathodes for gas rectifier tubes.

For certain types of gas rectifier tubes, more particularly those of the high current ratings, a cathode in the form of a cylinder of core metal oxide coated on its inner surface and surrounded by a heat shield is commonly used to afford the desired emissive surface. In certain applications and uses of such tubes, the power supply facilities available, make it desirable to heat the cathode indirectly by a suitable heater element, rather than by current flowing directly through the cathode, so that higher heating voltages may be employed.

Certain characteristics are desirable for such indirect heating elements for heat shielded oxide coated cathodes.

Among other things, the heater element should give a uniform heating of the cathode emissive surface, otherwise there may be localized overheating and formation of hot spots which adversely affect the useful life of the cathode. Also, in the interests of long life, an oxide coated cathode should be brought up to its operating temperature before the tube is used to conduct current; and for many applications and uses of such tube it is important that the initial heating time should be short. In this connection, where the heat is transferred from the heater winding to the cathode by direct conduction, as distinctive from radiation through intervening space, the temperature gradient between the cathode and heater winding is small; and when the cathode is cold, the temperature and resistance of the heater winding is correspondingly low, so that the heating current and heat input to the cathode is initially high and gradually reduces to the normal operating level as the cathode heats up, thereby reducing the initial heating time by giving a higher average heat input to the cathode while it is being heated to its normal operating temperature.

Certain problems are involved in providing a structure having these desirable characteristics of uniform heating of a hollow cylindrical cathode by direct conduction of heat from a heater winding. Among other things, the various turns of a heater winding must of course be insulated for high temperature operation; and in general, insulating materials suitable for this purpose, such as aluminum oxide, do not have the mechanical strength and physical characteristics to stand up under the abrasion and flexure likely to exist under the operating conditions of jar, vibration, and the like, when wire with such insulation is merely wound around the cathode in contact with its outer surface. Also, such a heater winding will conduct heat directly to the cathode for only a small part of its total surface where the wire is in actual contact with the cathode surface.

With these and other considerations in mind, the principal object of this invention is to provide a simple structural organization for indirectly heated cathodes of the oxide coated and heat shielded type for gas tubes, which will provide uniform, rapid-and efficient heating of exten- 2 sive cathode surfaces, and also have the other desirable characteristics for satisfactory performance and long life.

Generally speaking, and without attempting to define the nature and scope of the invention, it is proposed to cover the turns of a heater wire with its refractory insulation covering, which is wound directly on the outer surface of a cylindrical cathode, with a layer of sprayed or vaporized metal, which not only affords the desired anchorage to hold in place the individual turns of the heater wire against movement under jar and vibration, but also facilitates the transfer of heat to the cathode by direct conduction to give a short initial heating time and efficient heating during operation.

Various other objects of the invention, and its characteristic features, attributes and advantages will be in part apparent, and in part pointed out, as the description progresses.

Although the cathode structure of this invention may take various specific forms, and may be used for various types of gaseous discharge tubes, it is convenient in discussing the nature of the invention to refer to some tangible physical embodiment, such as illustrated in the accompanying drawings for a typical grid control rectifier tube. In the interests of simplicity, these drawings are directed to a showing of the structural features of the cathode and its heat shield, without attempting to illustrate the anode, grid or other elements of a complete tube.

In the accompanying drawings, Fig. 1 is a general view, with some parts broken away and some parts shown in section, of a heat shielded cathode structure in a tube envelope.

Fig. 2 is a transverse section on line 2--2 in Fig. 1.

Fig. 3 is an enlarged fragmentary view of a portion of the cathode and one lead-in connection of its heater winding.

Fig. 4 is an explantory diagrammatic representation of one way in which the heater winding may be formed.

The heat shielded cathode structure illustrated comprises in general a cylindrical cathode K, having an oxide coating on its inner surface, a heater winding H attached by sprayed metal to the outer surface of this cathode, and a multiple wall heat shield HS surrounding and enclosing the cathode and its heater winding except for a discharge opening in one end.

In the particular structural arrangement of parts shown, cathode K comprises a cylinder 5 of nickel to which is welded adjacent its lower edge the peripheral flange of a circular bottom 6. The cathode cylinder 5 with the heater winding around it as later described preferably extends below the circular bottom 6, so that this bottom may be heated by heat radiated from such extended portion of the cylinder and its heater winding and provide the desired uniform heating of all of the emissive surface of the cathode. An annular top member 7 is welded to the upper edge of the cathode cylinder 5 to facilitate attachment of the heat shield HS.

The inside surface of the cathode cylinder 5 and its bottom 6 is provided with one of the well known oxide coatings indicated at 8. It is assumed that this oxide coating will be formed in accordance with well known principles and practices of the art from the appropriate mixture including oxides of barium, strontium, or the like. One suitable oxide coating of the barium nickelate type suitable for this purpose is disclosed in the patent to D. V. Edwards et al., No. 2,081,864, May 28, 1937.

The heat shield HS in the arrangement shown comprises inner and outer cylindrical heat shield cans 10 and 11 of thin nickel, together with end pieces, to surround the cathode K completely, except for a discharge opening at one end, and provide an enclosed space around the cathode for the heater winding H. In the particular structural arrangementof .parts .shown, a pcripheral flange of a bottom end piece 12 is welded to the lower edge of the outer heat shield can 11. The inner heat shield can '10 is attached at its lower edge to -the bottom end piece 12 by a plurality of welded brackets 13, like the one shown. The upper edge of the inner heat shield can is welded to the peripheral flange of atop end piece 14 having therein a discharge opening indicated at 15 of the appropriate dimensions for the size of the cathode. This secondary top end piece 14 is also welded to the annular top member 7 of the cathode, so that the cathode, together with its heater winding H, is supported inside the heat shield HS from its top end piece 14. The upper edge of the outer heat shield can 11 is connected 'by a plurality of Welded brackets 16, like the one shown, to the upper'en'd of the innerheat Shield can 10. A number ofpimpled sheets of'thinnickel (not shown") may be included in the'space between the inner and outer heat shield cans 10 and 11 to add to the heat insulati'ng efliciency of the heat shield HS.

The heat shield HS, together with the enclosed-cathode K and its heater winding H, is supported in the tube envelope in a suitable manner. In the arrangement shown, it is assumed that the glass envelope E will have a circular mounting stem-18 fused at its lower edge to the body of the envelope, as indicated at 1'9, and thatthe heat shield HS is supported on this mounting stem 18 by a plurality of legs, preferably three in number for a three-point support, similar to the leg 20 shown. The bent upper end of each leg 20 is welded to the underside of the bottom end piece 12'of the heat shield HS, and is anchored in the usual manner in the mounting stem 18. One of these legs 20 extends through a gas-tight seal to the outside of the tube envelo'pe to provide'an external cathode connection. The circular mounting stem 18 is provided with the usual exhaust tubulation indicated at 21 for evacuating the tube envelope and introducing the gas filling in accordance with well knoWn practice.

Considering now the structural organization of this invention for indirectly heating the cathode cylinder 5, the heater winding H comprises a wire 25 of tungsten or like refractory metal, which is covered with a coating or layer of a refractory insulating material, indicated at 26, such as the conventional aluminum'oxi'de coating sintered 0n the tungsten wire in a suitable manner in accordance with recognized practice. This Wire 25 is wound with appropriately spaced turns directly ontheoutside of the cathode cylinder 5. Since this winding is 'to be connected to lead-in connections at the 'lo'wer'end of the cathode for supplying current to the heaterwinding,'it is convenient to form a length of the heater wire in a loop and wind this loop spirally on the cathode cylinder 5 from the top'down, with "substantial equal spacing between the severalturns of'wire, in the manner diagrammatically represented in Fig. 4. Wind'in'gthe heater wire double in this manner'gives a coil of uniform thickness with terminals at the same end; but any other suitable coil structure may be emloyed which 'aifords'the appropriate spacing of turns for uniform heating.

In'the structure of this invention, after the heater wire 25 with its coating of insulation '26 has'been wound on the outside surface of the cathode 'cylinder'5, a layer or coating of nickel or other suitable 'metal'is applied by any one of the well known metalspraying or vaporizing processes to cover the 'wire, and'a'lso form a bond with the cathode cylinder in the spaces between the wire turns. Such a metal coating, roughly indicated at 27, is conveniently applied by a metal spraying torch or .gun of the conventional type used in the well knownprocess of metal spraying. The metal, 'such as nickel, is sprayed over the surface in the usual way to' form a coating of a thickness to have suflicient mechanical strength to hold the turns of wire in place under jar and vibration, and aiford heat conductivity suitable for maintaining the cathode at its emissive temperature at a relatively low temperature gradient between it and the heater winding. In applying such a metal coating it is desirable to clean the surface of the cathode cylinder 5 by appropriate degreasing and cleansing agents before the heater Winding and its -metal coating are applied, and prepare the surface of the nickel ordinarily used for the cathode cylinder for close bonding of the sprayed metal by roughening such surface by sand blasting or the like.

The appropriate thickness of the sprayed metal coating and spacing of the turns of course depends upon the dimensions of the cathode and other factors; and it should be understood that the showing in the accompanying drawings is for explanatory purposes and is not intended to represent any particular 'proportioning of the parts. Generally speaking, it is contemplated that the thickness of the sprayed metal coating, constituting in effect a metal shell around the turns of the heater winding and bonded in direct contact with the outer surface of the cathode cylinder, will be selected to afford the appropriate mechanical strength an'd'heat conductivity. By way of explanation, and withoutlimiting the invention, a metal coating of a thicknessin the order of .001 inch is suitable for certain cathode structures. With regard to'the spacing of the turns of the heater winding, since heat is conducted from 'each turn through the sprayed metal shell to an adjoining area of the cathode surface at a relatively low temperature gradient, the desired uniform heating of "the cathode emissive surface may be obtained by turns spaced relatively far apart, the number of 'turns and size of wire being of course selected to conform with the heater voltage employed, amount of'heat required, and other factors likely to vary withthe particular design of tube.

In the structure illustrated for providing lead-in connections to theterminals of the'heater winding H, "a pair of lead-in rods 30, only one shown, is sealedin'the usual manner in the mounting stem 1-8 and extend through tubular insulators 31 of steatite or like heat resistant insulatingmaterial fitted. in holes in the bottom end 'piece 12 of'the heat shield HS. A bare end of the heater wire 25 is welded to a small angle bracket 32, which inturn is welded to the upper endof the associated lead-in rod 30. Each insulatorfai has an enlarged head or collar resting on the bottom end piece 12 of the heat shield, and is kept in place by the angle bracket 32 welded to its associated rod 30.

A metal sleeve or tube 34 around eachlead-in rod 30 is attached at its upper end to the bottom end piece 12 of the heat. shield by a welded lip or the'like, and fitsat its lower end around a boss or enlargement of glass, formed at the seal for therod 30. Thelea'd-in rods 30 are thus shielded by the metal sleeves 34 at cathode potential from the gas filling of the tube, so that nogaseous discharge can take place between these rods, although theirdifference in potential for theheater voltageemployed isabove the level for initiating such a discharge.

In this connectiomst-he exposed surfaces at .theelectrical'connection between the upper ends of lead-in rods 30- and the heater winding H :are located in. the enclosed space betweenthe cathode. cylinder 5 and the inner heat shield. can a 10, and are thus. eifectively isolated. from! the main gasifilling and any condition of-ionizationuit may assume during operation. The gas filling maycexistfin this enclosedspace, but being isolatedfrom' the mainerc discharge occurring in operation, its state of ionization is such that no'gaseous discharge will be initiated between the exposed'surfaces at the ends of the lead in rods at the distance of their separation and the gas pressure used by the voltages suitable for the heater winding.

From the foregoing explanation it can be readily seen how the indirectly heated cathode structure of this invention provides the desired rapid and uniform heating of extensive cathode surfaces in the heat shielded cathodes for gaseous discharge tubes, together with the other advantages and attributes in the way of efiicient heating by direct conduction through metal, and maintaining the individual turns in the heater winding in place against jar and vibration injurious to its insulation, all in a simple structure readily and economically fabricated.

The indirectly heated cathode structure of this invention may of course take various forms and shapes, and be employed with different structural arrangements of heat shields and the like, in gaseous discharge tubes of various types; and it should be understood that various modifications, adaptations and additions may be made in the particular structure illustrated without departing from the invention.

What I claim is:

1. An indirectly heated cathode structure for gaseous discharge tubes comprising, a hollow cathode of relatively thin core metal having an emissive oxide coating over its inside surface, a heater winding of closely spaced turns of heater wire covered with a refractory insulating material, said winding being around and in contact with the outer surface of said cathode, and a metalized layer of finely divided particles over said heater Winding and closely bonded to the intervening exposed surface areas of said cathode, said layer of metal fixing all parts of the heater winding to the cathode and being capable of conducting heat directly to the cathode core metal from the entire circumferential surface of the heater Wire at a small difference in temperature, thereby shortening the initial heating time for the tube and reducing the temperature of the heater wire for normal operation.

2. An indirectly heated cathode structure for gas tubes comprising, a hollow nickel cathode of relatively thin core metal having an emissive coating on its inner surface, said cathode having an outer surface area roughened for cohesion with metal particles, a heater winding of Wire covered with a refractory insulating material around and in contact with said roughened surface area of the cathode, and a layer of cohesive nickel particles over said heater Winding, said metal layer being bonded to the exposed parts of said roughened surface area and fixing all parts of the heater Winding to the cathode, said metal layer also being capable of conducting heat from the entire surface of the insulated heater wire to said cathode.

3. An indirectly heated cathode structure for gas tubes comprising, a hollow cathode of relatively thin core metal in the form of a cylinder with a closed bottom located intermediate its ends, said cathode having an emissive oxide coating on its inner cylindrical surface and the inner surface of said bottom, a heater winding of refractory insulated wire around and in contact with the outer cylindrical surface of said cathode throughout its length, said closed bottom being located to be heated by radiation from the end portion of the cylinder heated by said heater winding, and a layer of cohesive metal particles incrusting said heater winding and bonded to the intervening exposed surface of the cathode, said layer of metal transferring heat to the cathode from the entire surface of the heater Wire by direct conduction through metal as Well as rigidly holding all parts of the heater winding in place and protecting its insulation.

4. An indirectly heated cathode structure for gas tubes comprising, a hollow cathode of core metal having an emissive coating on its interior surface, a heater winding comprising a length of refractory insulated Wire doubled into a loop and spirally wound around and in contact with the outer surface of said cathode, said heater winding having a substantially uniform thickness with both terminals at the same end of the winding, and a layer of cohesive metal particles over the heater winding and bonded to the intervening exposed areas of the cathode outer surface, said metal layer fixing said heater Winding in place against displacement of its turns and also protecting its insulation against abrasion, said metal layer also affording short heat conducting paths through metal to the emissive coating from the entire circumferential surface of the heater wire.

References Cited in the file of this patent UNITED STATES PATENTS 2,065,997 Edwards et al. Dec. 19, 1936 2,459,997 Edwards et al. Jan. 25, 1949 2,497,911 Reilly et al. Feb. 21,1950 2,512,538 Baker June 20, 1950 2,572,881 Rothstein Oct. 30, 1951 2,650,997 Watrous, Jr. Sept. 1, 1953 FOREIGN PATENTS 477,078 Great Britain Dec. 21, 1937 

